Mean Aerodynamic Chord

[johnriketes]

Hi all

I have a weigh and a moment. How do I obtain the %MAC? Do I need anything more? Assume 0 moment is 25% MAC.

Has anyone got a formula to calculate %MAC?

Thanks in advance.

[hawk37]

Haven't done this for awhile, but....

You would need the length of the MAC, call it X. Take the total moment for the aircraft, and divide by the total weight. This gives you your arm, from the datum (reference point where all arms are taken from). I presume you mean the datum is defined to be at 25% mac. Then, if forward of the datum is defined to be positive, and if your arm is positive, the %mac is 25-(arm*100/X). Think this works.

[99jolegg]

I think all you need is the centre of gravity position (whether it be a distance from a the datum or a body station) and the location (again, distance from the datum or a body station) of the leading and trailing edge of the wing. Given the last two parameters, you know the length of the MAC.

I assume you're converting a CG position aft / forward of a datum to the %MAC?

% MAC = CG Position - LE MAC
********(TE MAC - LE MAC)

I think that works?

[johnriketes]

Thanks for the input guys. Helped settle a disagreement.

[barit1]

% MAC = CG Position - LE MAC
********(TE MAC - LE MAC)

Well, yeah, except the result will be a number between zero and one. Multiply by 100 to obtain %MAC of course.

[john_tullamarine]

Main thing to keep in mind is that %MAC is just another way of specifying the CG .. no more, no less. Of significance to the flight test and certification folk but not so for the line pilot.

[barit1]

%MAC is just another way of specifying the CG

...or any other feature of interest associated with the airfoil (e.g. lofting coordinates, flap or aileron hinge axis,...)